Prestressed concrete plank construction



W. C. HART FRESTRESSED CONCRETE PLANK CONSTRUCTION Nov. 8, 1966 5 Sheets-Sheet 1 Filed Jan. 22, 1964 INVENTOR fii m f (3 H4167 ATTORNEYS Nov. 8, 1966 w. c. HART PRESTRESSED CONCRETE PLANK CONSTRUCTION 5 Sheets-Sheet 2 Filed Jan. 22, 1964 \L wk W W x L m% m u I wfi, 4 H H E 3h m w A lh |M W wfi mws BY A.

Nov. 8, 1966 w. c. HART 3,

PRESTRESSED CONCRETE PLANK CONSTRUCTION Filed Jan. 22, 1964 5 Sheets-Sheet 3 Nov. 8, 1966 w. c. HART 3,233,457

PRESTRESSED CONCRETE PLANK, CONSTRUCTION Filed Jan. 22, 1964 5 Sheets-Sheet 4 a? INVENTOR 3/5 haw WW: [/ff/nzr ATTORNEYS Nov. 8, 1966 w. c. HART 3,283,457

PRESTRESSED CONCRETE PLANK CONSTRUCTION Filed Jan. 22, 1964 5 Sheets-Sheet a INVENTOR 6292/87 m ATTORNEY-S United States Patent The present invention relates generally to prestressed concrete and, more particularly to a new and improved method and apparatus for forming prestressed concrete slabs or planks, as well as a new and improved prestressed concrete plank construction. I

There is disclosed in the United States patent to Vander Heyden, No. 2,696,729, a method of forming concrete slabs wherein reinforcing rods are carried in aligned, opposed channels or passages in a plurality of concrete blocks. The blocks are placed under temporary compression by tensioning a clamping rod extending through aligned openings in the plurality of blocks and the channels containing the reinforcing rods are filled with grout along this stressed row of blocks, and the grout is allowed to set while the clamping rod is maintained under temporary tension. After the grout has set so as to bond the reinforcing rods to the blocks, the tension on the clamping rod is released and the clamping rod removed to thereby transfer the tension to the bonded reinforcing rods and complete the reinforced slab construction. I

Likewise, means for introducing grout under pressure in these opposed longitudinal channels or passages from the ends of the row are taught in the United States patent to Car-per, No. 2,897,570. While such prior art method and means for forming concrete slabs have enjoyed varying degrees of success, such a teaching necessarily is tedious, expensive and time consuming, requiring excessive clamping rod handling and storage facilities.

Remembering that the degree or portion of the initial prestress of a concrete plank is lost due to plastic flow, creep stress and the like, the above described prior art provisions for forming a prestressed plank of preformed concrete blocks necessarily provide only a nominal degree of prestressing. When the only stress placed on the plank is by the tension elements, which is retained after the initial compression of the plank is relieved, the resulting degree of prestress is inherently relatively small.

In addition, because of the relatively low tensile strength of the mild steel bar or rod stock employed as reinforcing elements in the prior art, it was frequently necessary to employ numerous relatively expensive, relatively large diameter rods in order to obtain the desired amount or degree of prestress inthe finished concrete plank. Moreover, the time consuming arrangement of such relatively expensive rods within the plank further increased the costs of manufacture.

Additionally, prior reinforced concrete plank constructions frequently have resulted in undesirable stress patterns and concentrations which caused splitting or cracking of the finished concrete plank. Also, the rods in the row of blocks in this prior art are clamped at both ends of the row to impart tension thereto, resulting in additional time-consuming assembly requirements.

According to this invention, the short-comings of the prior art are obviated by initially compressively stressing a preformed row of concrete blocks by imparting tension to one or more lengths of cable passed through the row of blocks. As will be more fully described hereinafter, the reaction force resulting from such tensioning of the cable will place the plank formed from these aligned blocks under the desired compressive stress. By virtue Patented Nov. 8, 1966 of this, a notably superior prestressed concrete plank is provided which does not rely on relaxing a compressed row of blocks as the sole means for imparting a prestress to the plank via tension rods.

This invention generally contemplates prestressed concrete planks formed from a plurality of specially constructed concrete blocks, each having centrally located apertures or holes which, when aligned, define a longitudinally extending cable receiving passage wherein one or more lengths of cable are passed through the passage from end to end of the block row and then tensioned so as to place the row of blocks under compression. Preferably, each of the concrete blocks is additionally provided with laterally spaced opposed holes which, when the blocks are aligned, define laterally spaced longitudinally extending opposed passages wherein reinforcing members such as steel rods are disposed. Thereafter the cable containing passage and the reinforcing member containing passages are filled with grout which is allowed to set while the cable is maintained under tension. Most desirably, certain spaced apart blocks along the row have transverse passages communicating with the longitudinal passages so as to receive grout under pressure along the length of the block row, from a suitable apparatus. To this end, a novel cable guide and tensioning arrangement are provided to temporarily stress the cable.

Accordingly, a principal object of the present invention is to provide a new and improved prestressed concrete plank construction.

An additional object of the present invention is to provide a new and improved method of making a prestressed concrete plank embodying my novel construction.

A further object of the present invention is to provide a new and improved apparatus for making a prestressed concrete plank embodying my novel construction.

A further object of the present invention is to provide an improved prestressed concrete plank construction having the desired degree of prestress without the necessity of employing relatively expensive and numerous relatively large diameter reinforcing rods.

A still further object of the present invention is to provide a new and improved method of making a prestressed concrete plank from a plurality of aligned concrete blocks which is relatively inexpensive and eliminates the need for employing separate clamping rods or the like to apply temporary initial compressive stress to the aligned blocks.

An additional object of the present invention is to provide a new and improved prestressed concrete plank construction comprising a plurality of aligned preformed concrete blocks having a transversely centered longitudinally extending passage wherein prestressed cable portions are positioned within the passage so as to provide the plank with enhanced load carrying capabilities.

Another object of the present invention is to provide a new and improved prestressed concrete plank construction comprising a plurality of aligned preformed concrete blocks having a transversely centered longitudinally ex tending passage wherein a prestressed cable is positioned within the passage in a generally parabolic or arcuate curve to provide the plank with enhanced load carrying capabilities.

A further object of the present invention is to provide a novel method of making a new and improved prestressed concrete plank construction comprising a plurality of aligned preformed concrete blocks having a transversely centered longitudinally extending passage ,wherein a'prestressed cable is positioned within the passage in a generally parabolic or arcuate curve to provide the plank with enhanced load carrying capabilities.

Another object of the present invention is to provide a new and improved prestressed concrete plank construction 3 which eliminates splitting or cracking of the plank resulting from undesirable stress concentrations.

A still further object of the present invention is to Other objects and advantages of the invention will become apparent from the following description and accompanying drawings in which:

FIGURE 1 is a perspective view showing the prestress- 1 ing of a concrete plank according to this invention;

FIGURE 2 is a sectional view taken along the line 22 3 of FIGURE 1;

FIGURE 3 is a perspective view of a concrete block i employed in forming a prestressed concrete plank according to the present invention;

FIGURE 4 is a perspective view of a tie plate which is employed in the prestressed concrete plank shown in FIGURE 1;

FIGURE 5 is a plan view, partly in section, of the rear cable guide assembly employed in the prestressing of the concrete plank shown in FIGURE 1;

FIGURE 6 is a side view, partly in section, of the rear 1 cable guide assembly shown in FIGURE 5;

FIGURE 7 is an enlarged perspective view of the front cable guide assembly and tensioning means employed in the prestressing of the concrete plank shown in FIG- URE 1;

FIGURE 8 is a longitudinal sectional view of the con- 1 crete plank shown in FIGURE 1 prior to prestressing';

FIGURE 9 is a longitudinal sectional view of the concrete plank shown in FIGURE 1 after prestressing and grouting;

FIGURE 10 is a longitudinal sectional view of the prestressed concrete plank shown in FIGURE 9 after the grout has set and the front and rear cable guide assemblies have been removed;

FIGURE 11 is a longitudinal sectional view showing a modified form of the present invention;

FIGURE 12 is a side view showing another modified form of the present invention;

FIGURE 13 is a perspective view, partly in section, illustrating still another modified form of the present invention;

FIGURE 14 is a side view showing a further modified form of the present invention;

FIGURE 15 is -a perspective view, partly in section, showing another modified form of the present invention;

FIGURE 16 is a plan view showing a still furthermodified form of the present invention;

FIGURE 17 is an end view of another modified form of the present invention;

FIGURE 18 is a plan view, partly in section, of a modified rear cable guide assembly employed in forming a prestressed concrete plank according to the present invention;

FIGURE 19 is a side view,'-partly in section, of the modification shown in FIGURE 18;

FIGURE 20 is an end view showing a modified means for anchoring the closed end of the cable during tensioning when the modification illustrated in FIGURES 18 and 19 is employed; and

FIGURE21 is a plan view of the. modification shown in FIGURE 20.,

Referring now to FIGURE 1 of the drawings there is shown a stationary conveyor assembly 10 including a plurality of transversely extending, longitudinally spaced apart rotatably..mounted. rollers 12. A plurality of sub- 8 stantially identical preformed concrete blocks 14 are arranged with their end faces 16 abutting and in aligned relationship on the rollers 12 according to the length of the slab or plank 1 8 to be formed.

.tudinally extending passages.

4 As best seen in FIGURES 2 and 3, each block 14,

which is made of suitable cementitious material, is generally rectangular in shape and includes substantially fiat and parallel upper and lower faces 20 and 22, respectively, substantially parallel end faces 16 and side faces 24 and 26 provided with suitable tongue and groove formations 28 and 30, respectively. Preferably, each block 14 is provided with two transversely spaced enlarged passages 32 extendingthrough the block from one end face 16 to the other and separated by a transversely centered vertically extending web portion 34. The enlarged passages 32 not only reduce the weight of the finished plank, but also define longitudinally extending passages for receiving electric conduits, heating ducts and the like having obvious utility where the formed planks are to be used in floor construction. In addition, each block 14 will be apertures 38 and 40 spaced transversely on either side of the central aperture 36. Each of the apertures 36, 38

and 40 extends through the block 14 from one end face to the other and is adapted to register with a corresponding aperture in adjacent blocks when arranged in plank forming alignment so'as to define generally parallel, longi- Preferably, the central aperture 36 is of a generally triangular configuration in cross section while the laterally located apertures38 and 40 are of a substantially circular configuration in cross section, as best seen in FIGURES 2 and 3.

Certain spaced apart blocks 14' (FIGURES 1 and 2) have vertical passages 42, 46 and 48 extending from the upper face 20 of the block 14' and communicating with the apertures 36, 38 and 40, respectively, for reasons to be more fully described hereinafter.

If desired, tie plates 50 can be interposed between each endmost pair of blocks 14 in order to provide thefinished plank with enhanced resistance to longitudinal cracking. The tie plates 50, which are preferably of sheet metal construction, have a pair of enlarged openings 32' and smaller openings 36', 3 8 and 40' corresponding to, and adapted to register with, the enlarged passages. 32 and .apertures 36, 38 and 40, respectively, in the adjacent blocks 14. The tie plates 50 may also be provided with raised boss portions 52 surrounding the openings 36', 38' and 40.

After the blocks 14 are arranged in plank forming alignment, as shown in FIGURE 1, a rod member 54, having a length substantially corresponding to the length of the plank 18 to be formed, is positioned in each of the passages defined by the aligned laterally located apertures 38 and 40. Each rod member 54, which is preferably a steel reinforced rod, is of a diameter substantially less than the diameter of the passage in which it isreceived in order to provide a grout receiving space. A

As best seen in FIGURES 1, 5 and 6, a rear cable guide assembly 56 is positioned adjacent the outwardly facing end face 16 of the rear endmost block 14 at one end of the row of blocks. The rear cable guide assembly 56 generally includes a rear channel-shaped abutment member 58 having .a substantially flat web portion 60 connecting the spaced flanges 62. The flat web portion '60 is provided with an aperture 64 adapted to register with the central aperture 36 in the rear endmost block 14 when the member 58 operatively abuts the outwardly facing end face 16 of the block. A tubular cable looping member 66, having the general form of a clos'edyloop when viewed in plan, is mounted on the outwardly facing surface of the web portion 60 in alignment with the taper-- ture 64 so as to be in operative cable receiving communication with the passage defined by the central. apertures 36 in the, blocks 14, The tubular cable looping member 66 includes a substantially straight, common tubular portion 68 extending perpendicularly outward of the web portion 60 and having a laterally disposed cable access aperture 69 and a tubular loop portion 70. The tubular loop portion 70, as best seen in FIGURE 5, includes two tubular branch portions 72 in communication with the bore of tubular portion 68 which, as they extend outwardly thereof, initially diverge and then arcuately converge to meet at their outermost extent and thereby form a closed tubular loop. The tubular cable looping member 66 will thus be seen to define a closed loop-like tubular passage in communication with the passage defined by the central apertures 36 in the blocks 14. Preferably, the cable looping member 66 is provided with strut members 74 and 75 to provide enhanced structural support, as will be readily appreciated.

If desired, the web portion 60 of the channel-shaped abutment member 58 may be provided with two additional apertures (not shown) transversely spaced so as to register with the laterally located apertures 38 and 40 in the block 14 to thereby permit introduction of the rod members 54 after operatively positioning cable guide assembly 56.

A front cable guide assembly 76 is positioned adjacent the outwardly facing end face 16 of the foremost block 14 of the row, preferably adjacent an end of the conveyor 10, as best illustrated in FIGURES 1 and 7. The front cable guide assembly 76 includes a channel-shaped front abutment member 78 having a substantially flat web portion 80 connecting the spaced flanges 82. The flat web portion 80 is provided with closely spaced apertures 84 and 86 adapted to register with the adjacent central aperture 36 in the foremost block 14 when the member 78 operatively abuts the outwardly facing end face 16 of the block. Alternatively, instead of the two spaced apertures 84 and 86, a single, somewhat larger aperture (not shown), adapted to register with the central aperture 36, may be provided in the web portion 80. A pair of transversely spaced vertically extending plates 8-8 and 90 abut the outwardly facing edges of the flanges 82 of the channel-shaped member 78. Preferably, the plates 88 and 90 are joined to the channel-shaped member 78 by welding or the like. Also abutting the outwardly facing edges of the flanges 82 of the channel-shaped member 78, intermediate the ends thereof, is a vertically extending plate member 92 having apertures (not shown) axially aligned with the apertures 84 and 86 in the web portion 80. Preferably the plate 92 is joined to the channelshaped member by welding or the like. The plate 92 is preferably reinforced by a horizontally extending plate 94, as best seen in FIGURE 7.

With the front cable guide assembly 76, the blocks 14, the tie plates 50, and the rear cable guide assembly 56 positioned as shown and described, the leading end 96 of a cable 98, preferably steel, is passed through one of the apertures in the plate member 92 and the corresponding aligned aperture, such as 84, in the web 80' of the front abutment member 78, through the longitudinal passage defined by the central apertures 36 in the blocks 14, through the aperture 64 in the rear abutment member 58 and into the common tubular portion 68 of the tubular cable looping member 66. Continued longitudinal feeding of the cable 98 will cause the leading end 96 to pass into one of the tubular branch portions 72 where it will be guided in a generally U-shaped path to form a loop and enter the other tubular branch portion 72 and thence return through the common tubular portion 68, the aperture 64, and the passage defined by the central apertures 36 in the blocks 14 until the leading end 96 emerges from the other aperture 86 in the front abutment member 78 so that the opposed leading end 96 and trailing end 100 extend through and outwardly beyond the vertically extending plate 92 of the front cable guide assembly 76. Thus it will be seen that two lengths of cable will be disposed in the passage defined by the central apertures 36 in the blocks 14.

As best seen in FIGURES 1 and 7, there is provided a means 102 for tensioning the cable 98 by pulling the cable ends 96 and 100. To this end, a plate 104 has attached to its upper surface intermediate its ends, cylindrical, internally threaded sleeves 106 and 108, the sleeves being axially aligned with the apertures 84 and 86, re-

spectively, in the front abutment member 78 so that the cable ends and 96 may be operatively received so as to extend through the sleeves 106 and 108, respectively, Threadedly received in the respective sleeves 106 and 108 adjacent the vertical plate member 92, are externally threaded collars 110 and 112, having knurled heads 114 and 116, respectively, and longitudinal bores or passages adapted to freely receive the respective cable ends 96 and 100.

Next, vice members or cable clamps 118 and 120 are slid over the respective ends 100 and 96 of the cable 98, these clamps 118 and 120 being in abutting relationship with the outer ends of the respective sleeves 106 and 108, as best shown in FIGURE 7. The cable clamps 118 and 120 are of conventional construction, and each in cludes a tapered outer sleeve 122 and a gripping surface 124 carried by a cooperatively tapered collar 126 of split construction.

Extending from the front end of the row of blocks defining the plank 18 is a track 128, carrying a wheeled cart 130. Fixed to the cart 130 is a fluid cylinder 132 having a piston rod 134 extending therefrom toward the front end of the row of blocks. Pivotally connected to the free end of the piston rod 134, as by a removable pin 136, is a clamping assembly 138 having transversely spaced perpendicularly depending end members 140. By virtue of the pivotal connection of the clamping assembly 138 to the piston rod 134, the vertically extending end members 140 may conveniently be pivoted into and out of the space between the laterally extending inner edge of the plate 104 and the front cable guide assembly 76. The cart 130 also carries transversely spaced opposed longitudinally extending bars or struts 142 positioned for abutment at their ends with the outwardly facing surface of the front cable assembly 76, for reasons to be more fully apparent hereinafter.

With the elements assembled as described, the cart 130 is moved toward the front end of the row of blocks and the end members 140 of the clamping assembly 138 are pivoted downwardly into the space between the sleeve carrying plate 104 and the front cable guide assembly 76, as aforedescribed. The fluid cylinder 132 is then actuated to move the piston rod 134 away from the row of blocks, so as to effect firm engagement of the end members 140 with the inner edge of the sleeve carrying plate 104 and thereafter exert tension on the cable 98 by pulling the cable ends 96 and 100 through the cooperation of the sleeves 106 and 108 and the cable clamps 118 and 120, the reaction force being transmitted to the row of blocks through the longitudinally extending struts 142 carried by the wheeled cart 130. This movement of the piston rod 134 inwardly of the fluid cylinder 132 causes the cable clamps 118 and 120 to firmly grip the cable ends; and, when a predetermined tension is imparted to the cable, which is preferably controlled automatically by a pressure responsive solenoid operated valve (not shown) operatively connected to the fluid source (not shown), the collars 110 and 112 are unthreaded from their respective sleeves 118 and 120 until they abut the outwardly facing surface of the vertical plate member 92. Thus, the tension imparted to the cable 98 and the consequent compression imparted, as a reaction thereto, to the aligned row of blocks is maintained by engagement of the collars 110 and 112 with the plate member 92. The fluid cylinder 132 is then deactuated to permit disengagement of the end members 140 with the sleeve carrying plate 104 by upward pivotal movement of the clamping assembly 138.

The row of blocks forming the plank 18, which is now subjected to the desired predetermined degree of prestress, may he moved along the rollers 12, as desired, away from the wheeled cart to permit the same to be ema 7 ployed for the prestressing of another row of blocks. With the row of blocks forming the plank 18 so stressed, 1 grout 144 is introduced into the longitudinally extending rod member containing passages defined by the apertures 38 and 40 and the longitudinally extending stressed cable lcontaining passage defined by the central apertures 36, gthrough the vertical passages 42, 46 and 48 in the blocks :14' disposed along the length of the row of blocks. By ;'introducing the grout 144 under pressure through the ,apertures 42, 46 and 48in the blocks 14, the longitudinal passages are filled quite rapidly and efficiently. The grout 144 or other suitable bonding substance which is employed should be capable of providing a strong :stress-transmitting bond between the stressed cable 98 and the concrete blocks 14.

After the longitudinally extending passages defined by the apertures 36,38 and 40 have been filled with grout, the plank 18 composed of the compressively stressed blocks 14 is permitted to set a sufiicient time to allow the grout to set thereby fixing the cable 98 under stress 'within its longitudinally extending passage as Well as operatively securing the rod members 54 within their respective longitudinally extending passages. Thereafter, the front and rear cable guide assemblies 76 and 56,'respectively, are removed as by burning or cutting the exposed portions of the cable 98 at the front cable guide assembly 76, between the vertical plate member 92 and the web portion 80 of the channel-shaped member 76, and at the rear cable guide assembly 56 through the cable access aperture 69 in the cable looping member 66. As will be apparent, the sleeves 106 and 108, collars 110 and 112 ,and cable clamps 118 and 120 may be reused after removal of the severed cable ends 96 and 100 therefrom.

The tendency of the compressed concrete blocks 14, in the finished prestressed plank 18, to expand after removal of the front and rear cable guide assemblies 76 and 56, respectively, is resisted entirely by the grouted tensioned cable 98 without any transfer of tension to the grouted rod members 54. Thus, the aforementioned limitations and disadvantages inherent in the prior art wherein the initial prestress applied during forming of a prestressed plank is transferred to bonded reinforcing rods are effectively and simply overcome. The laterally disposed longitudinally extending grouted rod members 54 not only resist any tendency of the blocks 14 to twist relative to one another about the longitudinally extending transversely centered axis defined by the tensioned cable 98, but also provide the formed plank 18 with enhanced load carrying capabilities, as will be .more fully described hereinafter.

The tie plates. 50, of course, are retained within the formed plank 18 by the grouted and stressed cable 98 and the grouted rod members 54. Thus, it has been found that prestressed concrete planks 18 formed of blocks 14 and prestressed grouted cable 98, as described, retain a notably superior resistance to longitudinal cracking by provision of the tie plates 50 disposed adjacent the opposed end regions of the for-med plank 18 so as to distribute the local stresses throughout the cross sectional area of the plank 18.

Reference is now made to FIGURES 8 and 9 wherein there is shown the relative disposition of the cable 98 within the longitudinally extending passage defined by the central apertures 36 in the aligned blocks 14 and the disposition of the blocks 14 before, and after, respectively, the aforedescribed application of prestressing tension to the cable 98. In FIGURE 8, before the application of prestressing tension to the cable 98, the aligned blocks 14 will be seen to be disposed in a substantially straight condition with the cable 98 similarly disposed along the top surface of the longitudinally extending passage defined by the central apertures 36 in the blocks 14. As

best seen in FIGURE 9, application of the desired prestressing tension to the cable 98, which is disposed below the vertically centered longitudinal axis of the plank 18 8 adjacent the lower faces 22 of the blocks 14,will result in upward vertical displacement of centrally located blocks relative to the end blocks, thereby causing the plank 18 to assume a slight camber or arched configuration and thus dispose the cable 98 at the longitudinal central portion of the plank 18 adjacent the lower surface of the passage defined by the central apertures 36. At each end of the plank 18, the ends of the tensioned cable 98 will remain disposed adjacent the upper surface of the passage defined by the central apertures 36 since the tensioned cable 98 remains in a substantially straight con dition. The passage defined by the central apertures 36, which will be seen to have assumed the form of a positive curve of arcuate or parabolic configuration, is then filled with grout 144, which is allowed to set, as aforementioned. Thus, the cable 98 will be fixed within the curved passage defined by the central apertures 36 so that the end portions thereof are adjacent the upper surface of the passage at the ends of the plank, while the central portions of the cable 98 are adjacent the lower central surface of the passage.

After the grout has set andthe front and rear cable guide assemblies 76 and 56, respectively, have been removed, the plank 18 will be in the aforedescribed arched condition, as best seen in FIGURE 10. Due to the fact that the cable 98 has become set within the grout 144, the central portion of the cable 98 will be seen to be maintained adjacent the central lower portion of the central passage while the cut ends are adjacent the upper surface of the passage at the ends of the plank 18.

Referring now to FIGURE 11 of the drawings, there is shown a modified form of the present invention wherein a support member 146 having an arcuate supporting surface 148 is employed to support the prestressed and grouted plank 18 in a predetermined arched condition while the grout 144 is setting. This modification, employing the support member 146, is particularly useful where the amount or degree of tension applied to the cable 98 to prestress the plank 18 may not in and of itself effect the desired amount of camber or arching of the plank 18 and thus effect the aforedescribed desired disposition of the cable 98 during the setting of the grout 144. The arcuate supporting surface 148 will be seen to support the central portion of the plank 18 at a higher elevation than its ends so that the cable 98 at the longitudinal center of the plank 18 is closely adjacent the lower surface of the passage defined by the central apertures 36 While the cable ends are adjacent the upper surface of the passage at each end of the plank in a fashion substantially similar to the embodiment described by reference to FIG- URE 9. After the grout 144 has set, the front and rear cable guide assemblies 76 and 56, respectively, are removed as aforementioned, and the supporting member 146 is removed from the underside of the plank 18. The formed prestressed plank 18 will then relax somewhat so as to assume the lesser camber or arched configuration which is imparted by the tensioned cable 98 alone, substantially similar to that aforedescribed by reference to, and shown in, FIGURE 10. The tensioned cable will assume a slightly negative or upwardly concave curve with the cut ends of the cable adjacent the upper surface of the central passage and the central portion of the cable at the longitudinal center of the plank 18 closely adjacent the lower surface of the passage.

While the limited controlled arching or camber of'a prestressed concrete plank as aforedescribed is frequently desirable, it should also be noted that an CXCCfiiVfi arching or camber can impair the functioning of the prestressed plank. Such undesirable excessive arching of the plank may occur during formation of the plank as a result of the initial compression'of the blocks by the ten lieved to result from a slow but continuing shortening of the plank below its vertically centered, longitudinal axis due to the longitudinal compression of the plank imparted by the tensioned cable. Such long-term arching can ultimately impair the function of the prestressed concrete plank to such an extent that its removal and replacement will be necessary. In prestressed concrete planks constructed according to the teachings of the instant invention, such long-term arching has been effectively controlled, by virtue of the laterally spaced rod members disposed below the vertically centered longitudinal axis of the plank. It has been observed that a short time after the grout has set and the plank has been completed, the rod members, which are bonded to the plank by the grout, are under compression. It is believed that the rod members are thus effective to control and limit such longterm arching by resisting the tendency of the plank to shorten below its vertically centered longitudinal axis. Thus, it will be apparent that an effective, simple and inexpensive means of controlling long-term arching or long-term camber has been provided by employing the rod members 54, as aforedescribed.

Reference is now made to FIGURE 12 of the drawings wherein there is shown a modified form of preformed concrete block 114 which can be employed in the present invention to form a prestressed plank 118. The block 114 differs from the block 14, just described, in that the upper face 120 is slightly shorter than the lower face 122 so that the end faces 116 are not parallel but taper or upwardly converge as shown, greatly exaggerated, in FIGURE 12. As was the case with the blocks 14, the blocks 114 are provided, adjacent the lower face 122 thereof, with a transversely centered aperture 136 and laterally located apertures 140 spaced transversely on either side of the central aperture 136, which are adapted to align with corresponding apertures in adjacent blocks so as to define longitudinally extending passages. When the blocks are arranged in plank forming alignment, a

small upwardly opening wedge-shaped space 150 will be formed between the opposed end faces 116 of adjacent blocks 114. The use of blocks 114 having upwardly converging end faces 116 to form a prestressed plank 1118, as aforedescribed, will limit the amount of initial camber or arching of the plank which results from the tensioning of the cable and consequent compression of the aligned blocks. Thus, this modification utilizing blocks 114 having upwardly converging end faces 116 is particularly useful where prestressing would normally result in an excessive or undesired amount of camber or arching of the plank.

Reference is now made to FIGURE 13 of the drawings which illustrates another modified form of the present invention wherein a plurality of longitudinally aligned prestressed concrete planks 218, 218' and 218" are simultaneously formed. The embodiment of the invention shown in FIGURE 13 differs from that aforedescribed by particular reference to FIGURES 1-7, in that spacer members 202 are interposed between adjacent concrete blocks 214 at preselected positions along the row. Thus the disposition of the spacer members 202 relative to the front and rear cable guide assemblies 276 and 256, respectively, will determine the lengths of the planks 218, 218 and 218". The spacer members 202 are preferably metal plates bent or otherwise formed into the shape of a rectangle having longitudinally spaced vertical end walls 204 adapted to engage the opposed end faces 216 of adjacent blocks 214. Each vertical end wall 204 of the spacer members 202 is provided with a transversely centered aperture 206 and two laterally located apertures 208 spaced transversely on either side of the central aperture 206. The central aperture 206 and laterally located apertures 208 are adapted to align with the corresponding apertures 236, 238 and 240, respectively, in the adjacent blocks 14 when the spacer members 202 are operatively disposed along the row of aligned blocks.

Certain spaced apart blocks 214' have vertical passages 242, 246 and 248 communicating with the apertures 236, 238 and 240, respectively, to facilitate the introduction of grout under pressure into the planks 218, 218 and 218". In addition, tie plates 250, substantially identical to the aforedescribed tie plates 50, can be interposed between each endmost pair of blocks 214 in each of the planks 218, 218 and 218".

The plurality of prestressed planks 218, 218 and 218" are formed simply in a manner substantially similar to that described by reference to FIGURES 17. Thus, after the rod members 254 are positioned in that longitudinally extending lateral passages defined by the laterally located apertures 238 and 240 in the blocks 214, a

cable 298 is passed through the aligned central apertures 236 of the entire row of blocks 1214 which is segmented into planks 218, 218' and 218" by the spacer members 202. As aforedescribed, the looping member 266 of the rear cable guide assembly 256 will guide the leading end 296 of the cable 298 in a generally U-shaped path to form a loop whereby continued feeding of the cable'298 will return the leading end 296 through the cable receiving passage defined by the apertures 236 so that two lengths of cable will be disposed in each plank 218,218 and 218". Thereafter, prestressing tension is applied to the cable 298 to thereby place the blocks 214 forming the planks 218, 218' and 218" under prestressing compression and the cable 298 and rods 254 are then grouted in place, all in a manner substantially identical to that aforedescribed by reference to FIGURES l-7.

After the grout has set, the row is broken into its individual planks 218, 218 and 218" by burning or otherwise cutting the cable at its ends to remove the rear and front cable guide assemblies 256 and 276, respectively, as well as serving the exposed cable 248 and rod members 254 which are visibly accessible from the open sides of the spacer members 202. Thus, when the block row in FIGURE 13 is broken down, three slabs 218, 218 and 218" are formed by a single stressing operation on the whole row. As will be appreciated, the length of the rows and the number of slabs formed is readily adjusted by the particular demand, and special equipment or the like is not required to change the slab lengths as well as the number of slabs formed by a single cable stressing step, as previously described.

Turning now to FIGURE 14, there is shown a modified arrangement for obviating the transmission of undesirable stress along the end regions of the prestressed plank which cause a longitudinal cracking of the plank along the central vertical plan thereof due to the greater stress at the ends, which longitudinally compress the concrete at these localized end regions. There is shown here an end block 314 of the plank 318, having bevelled upper end lower cut-out portions 300 and 302, respectively, adjacent its inner end face 316. Transversely extending reinforcing rods 304 and 306 are positioned within these bevelled port-ion-s 300 and 302 respectively, and secured with grout 344. As the plank 318 is not subjected to these localized compression stresses during the grout setting stage due to the channel-shaped members of the front and rear cable guide assemblies, the grout can set while the grout introduced into the longitudinally extending passages is also setting. When the setting has been completed and the cable guide assemblies are removed, the undesirable local stress in the plank 318 is efficiently distributed throughout the surface thereof via the reinforcing rods 304 and 306 and their surrounding grout 344. Thus, the reinforcing rods 304 and 306 and grout perform an equivalent function to thatof the tie plates 50. Accordingly, the modification is equally applicable to the various other modifications described herein, as the end blocks in each prestressed plank may be so formed.

3 verse recesses 400 and 402 inwardly of the corners thereof, which carry reinforcing rods 404 and 406, respectively,

in grout. By virtue of this arrangement, the tendency of the endmost blocks of the planks to crack longitudinally, 1 as aforedescribed, is obviated. The end blocks 414 may I be conveniently formed prior to assembly of the plank or simultaneously with the formation thereof, as desired.

Ailso, depending on the degree of stress in the'cable 1 lengths, the block width configuration and so forth, the end blocks 314 (FIGURE 14) or 414 (FIGURE 15) may be formed with reinforcing at one end edge only, as necessary.

Referring now to FIGURE 16 of the drawings, there is shown another modified form of prestressed concrete plank 518 embodying the invention. The construction of 1 the prestressed concrete plank 518 differs from that of the prestressed concrete. plank 18, aforedescribed, in that the longitudinally extending passage defined by the transversely centered apertures 536 is provided with only a single length of tensioned cable 598. Since only one length of cable 598 is disposed within the longitudinal passage defined by the central apertures 536,

it is not necessary to employ my novel cable loop- 1 ing member 66, aforedescribed. Thus, the cable 598- i can be anchored at the rear end of the. plank 518 by means of a conventional cable clamp 500 embody- 1 portion 560 connecting a pair of spaced flanges 562.. The

3 flat web portion 560 is provided with an aperture 564 1 adapted to register wit-h the central cable containing aperture 536 in the rear endmost block 514 when the abut- 1 ment member 558 operatively abuts the outwardly facing end face 516 thereof. Abutting the outwardly facing edges of the flanges 562 of the channel-shaped abutment member 558 is a vertically extending plate member 566 having an aperture 568 axially aligned with the aperture 564 in the web portion 560 of the abutment member. The front cable guide assembly 576 is of a construction substantially identical to that of the aforedescribed front i cable guide'assembly 76, and includes an apertured channel-shaped abutment member 578, a pair of transversely spaced vertically extending pl-ates 588 and a transversely centered, operatively aligned apertured plate 592 abutting the channel-shaped abutment member. The means for tensioning the cable is substantially similar to the means 102, aforedescribed, and includes an internally threaded sleeve 506, an externally threaded collar 510 having a knurled head 512 and a cable clamp 520.

Prestressing of the plank 518 formed by the aligned row of blocks 514 is accomplished in a manner substantially similar to that aforedescribed by reference to FIG- URES 1-7, with the exception that only a single length of vertically extending plates 592 and 566. With the cable clamp 500 operatively engaging the cable end 596 and the sleeve 506, collar 510 and clamp 520 operatively positioned on the other cable end 594, the plank 518 is sub- 12 jected to the desired degree of prestress by tensioning the cable 598, as in the embodiment of the invention described by reference to FIGURES 1-7. The collar 510 is then unthreadedso as to abut the vertical plate member 592 I and thereby maintain the cable 598 under the desired prestressing tension and the blocks 514 under compression during introduction and setting of the grout in the longitudinally extending rod and cable containing passages. As in the aforementioned embodiments, the front and rear cable guide assemblies 576 and 556, respectively, are removed by severing the exposed end portions of the cable Reference is now made to FIGURE 17 of the drawings wherein there is shown a further modified form of a preformed concrete block 614 which can be employed to form a prestressed concrete plank according to the invention. The block 614 differs from the block 14, aforedescribed, in that the transversely centered aperture 636 adapted to receive the prestressing cable 698 has a vertically elongated generally rectangular configuration in cross section, the upper and lower edges 632 thereof preferably being curved or arcuate. This modified block construction 614 has particular utility where only a single length of cable 698 is disposed within the prestressed plank, as just described by reference to FIGURE 16.

Reference is now made to FIGURES 18, 19, 20 and 21 of the drawings, wherein there is shown a modified rear cable guide assembly 756'and a modified arrangement for anchoring or restraining the closed or looped end of the cable 798 during formation of the prestressed plank 718. The rear cable guide assembly 756 differs from the rear cable guide assembly 56, aforedescribed, in that the tubular cable looping member 766 is adapted to be removed after forming the cable 798 into a loop and prior to tensioning of the cable. To this end, the tubular cable looping member 76 is of a separable or. split construction and includes top and bottom sections 770 and 772 having mating flanges 774 and 776, respectively. The top and bottom sections 770 and 772 are held together in operative association by suitable removable fastener means such as bolts 778 which extend through the opposed mating flanges 774 and 776. The common tubular portion 768 is preferably provided with flanges 780, through which suitable fastenermeans, such as bolts 782, extend so as to removably secure the tubular cable looping member 766 to the web 760 of the rear channelshaped abutment member 758.: When assembled and operatively positioned adjacent the rear channel-shaped abutment member 758,the tubular cable looping member 766 will be seen to define a closed loop-like tubular passage in communication with the central cable receiving passage in the plank 718. After the cable 798 has been disposed within the central cable receiving passage in the plank 718 by being guided through the tubular cable looping member 766 in a generally U-shaped pathto form a loop, in a manner substantially similarto that de scribed by reference to FIGURES l-7, the bolts 778 and 780 are removed to thereby permit disassembly and removal of the tubular cable looping member 766.

As best seen in FIGURES 20 and 21, after the tubular cable looping member 766 has been removed from operative association with the closed or looped end of the cable 798 and the rear channel-shaped abutment memher 758, a bar member 784 is disposed within the now exposed, closed or looped end-of the cable 798. Upon the application of prestressing tension to the cable 798, as aforedescribed, the bar 784 will engage both the rear channel-shaped abutment member 758 and the closed end of the cable 798 so as to anchor or restrain the closed end of the cable to thereby permit tension of the same during the grouting operation. After the grout has set, the exposed, closed end of the cable 798 is severed, as by burning or cutting, so as to permit removal of the bar member 784 and the rear channel-shaped abutment member 758 from operative association with the plank 718.

it will thus be seen that the objects of this invention have been fully and effectively accomplished. It will be realized, however, that the foregoing specific embodiments have been shown and described only for the purpose of illustrating the principles of this invention and are subject to extensive change without departure from such principles. Therefore, this invention includes all modifications encompassed within the spirit and scope of the following claims.

What is claimed is:

1. A method of forming a pre-stressed structural unit comprising the steps of: aligning a plurality of preformed blocks in end to end abutting relationship to provide a row thereof, each of said blocks including a longitudinally extending transversely centered aperture and at least one longitudinally extending lateral aperture transversely spaced from said transversely centered aperture, said apertures being adapted to align with corresponding apertures in adjacent blocks when said blocks are aligned in end to end abutting relationship so as to define longitudinal passages therethrough; disposing a rod means in a substantially unstressed state in each passage defined by said lateral apertures; disposing a cable means in the transversely centered passage defined by said transversely centered apertures; tensioning said cable means and compressing said row of blocks longitudinally thereof; introducing stress transmitting bonding material into each of a said longitudinal passages; and maintaining said cable means under tension and said row of blocks under compression while said bonding material effects a stress transmitting bond between said row of blocks, said rod means in a substantially untensioned state, and said tensioned cable means, whereby said tensioned cable means will maintain said row of blocks under compression and said rod means will remain substantially untensioned.

2. The method defined in claim 1 whereinprior to effectuation of the stress transmitting bond by the bonding material the central portion of the plank formed by the row of blocks is deflected vertically relative to the end portions thereof so as to cause the plank to assume a generally arched configuration and thereby disposed the opposed end portions of the tensioned cable means within the transversely centered passage adjacent the upper surface of said transversely centered passage and dispose the central intermediate portion of said cable means adjacent the lower surface of said transversely centered passage and said cable means is maintained so disposed relative to said transversely centered passage while said bonding material effects said stress transmitting bond.

3. The method defined in claim 2 wherein the plank formed by the row of blocks is supported in its arched configuration by a correspondingly arcuate supporting surface while the bonding material effects the stress transmitting bond.

4. The method defined in claim 2 wherein the transversely centered cable means containing passage is disposed below the vertically centered longitudinal axis of the plank formed by the row of blocks so that the tensioning of the cable means effects the deflection of the central portion of the plank vertically relative to the end portions thereof so as to cause the plank to assume the generally arched configuration.

5. The method defined in claim 4 wherein the end faces of at least some of the blocks forming said row converge upward-1y so as to limit the amount of vertical deflection of the central portion of the plank relative to the end of portions thereof.

6. The method defined in claim 1 which includes the step of providing transversely extending reinforcing members adjacent each end of the plank formed by the row of blocks.

7. The method defined in claim 6 wherein the reinforcing members comprise tie plates positioned between each endmost pair of blocks.

14 8. The method defined in claim 6 wherein the reinforcing members comprise rod members grouted to each endmost block of the row.

9. A method of forming a prestressed plank comprising i the steps of: aligning a plurality of preformed blocks in end to end abutting relationship to provide a row thereof, each of said blocks including a longitudinally extending transversely centered apertured and a pair of longitudinally extending lateral apertures transversely spaced on either side of said transversely centered apertures, each of said apertures being adapted to align with corresponding apertures in adjacent blocks when said blocks are aligned in end to end abutting relationship so as to define longitudinal passages therethrough; disposing a rod member in a substantially unstressed state in each lateral passage defined by said lateral apertures; passing a cable through the transversely centered passage defined by said transversely centered apertures so that at least one length of cable is disposed within said transversely centered passage with portions of said cable extending beyond each opposed end of the plank formed 'by the row of blocks; maintaining the portion of said cable extending beyond one end of the plank fixed against longitudial movement relative thereto, gripping the portion of said cable extending beyond the other end of the plank and imparting tension to said cable to thereby exert compression forces longitudinally of the plank; introducing grout under pressure into said longitudinal passages and maintaining the cable under tension and the row of blocks forming the plank under compression while the grout is allowed to set so as to secure said rod members in a substantially untensioned state within their respective lateral passages and effect a stress transmitting bond between said tensioned cable and said plank whereby said tensioned cable will thereafter maintain said plank under compression.

10. The method defined in claim 9 wherein prior to the setting of the grout, the central portion of the plank formed by the row of blocks is deflected vertically relative to the end portions thereof so as to cause the plank to assume a generally arched configuration and thereby dispose the opposed end portions of the tensioned cable within the transversely centered passage adjacent the upper surface of said transversely centered passage and dispose the central intermediate portion of said cable adjacent the lower surface of said transversely centered passage and said cable is maintained so disposed relative to said transversely centered passage while the grout is allowed to set.

11. The method defined in claim 10 wherein the plank formed by the row of blocks is supported in its arched configuration by a correspondingly arcuate supporting surface while the grout is allowed to set.

12. The method defined in claim 9 wherein the transversely centered cable containing passage is disposed below the vertically centered longitudinal axis of the plank formed by the row of blocks so that the tensioning of the cable effects the deflection of the central portion of the plank vertically relative to the end portions thereof so as to cause the plank to assume a generally arched configuration.

13. The method defined in claim 12 wherein the end faces of each of the blocks converge upwardly so as to limit the amount of vertical deflection of the central portion of the plank relative to the end portions thereof.

14. The method defined in claim 9 which includes the step of providing transversely extending reinforcing members adjacent each end of the plank formed by the row of blocks.

15. The method defined in claim 14 wherein the reinforcing members comprise tie plates. positioned between each endmost pair of blocks.

16. The method defined in claim 9 wherein the cable is passed through the transversely centered passage so as to form a generally U-shaped loop whereby two lengths of cable are disposed within said transversely centered 1 5' passage with' the closed end of said U-shaped loop formed by said cable extending beyond one end of the plank and the open end of said U-shaped loop formed by the opposed free ends of the cable extending beyond the other end of the open end of said U-shaped loop are gripped by cable gripping means.

18. The method defined in claim 17 wherein tension is imparted to the cable and compression forces simultaneously exerted longitudinally of the plank by means of a power operated ram operatively connected to said cable gripping means and said other end of the plank.

19. The method defined in claim 9 wherein two lengths of cable are disposed within the transversely centered passage in the row of blocks.

20. A method of forming a prestressed plank comprising the steps of: aligning a plurality of preformed blocks in end to end abutting relationship to provide a row thereof, each of said blocks including a longitudinally extending transversely centered aperture and a pair of longitudinally extending lateral apertures transversely spaced on either side of said transversely centered aperture, each of said apertures being adapted to align'with corresponding apertures in adjacent blocks when said blocks are aligned in end to end abutting relationship so as to define longitudinal passages therethrough; disposing a rod member in an unstressed state in each lateral passage defined by said lateral apertures; positioning cable looping means in operative alignment with said transversely centered passage adjacent one end of the plank formed by the row of blocks; passing the leading end of a cable from the other end of said plank through said transversely centered passage, said cable looping means and back through said transversely centered passage so that said cable is formed into a loop having an open end portion defined by the leading and trailing ends of said cable and a closed end portion, said closed end portion of the loop being disposed within said cable looping means and said leading and trailing ends of said cable defining said open end of said loop extendingbeyond said other end of the plank whereby two lengths of cable are disposed within said transversely centered passage; gripping said leading and trailing ends of said cable; applying a tensioning force to said leading and trailing ends, said tensioning force being directed away from said closed end portion of said loop to impart tension to said cable; transmitting the opposed reaction force to said plank to thereby exert compression forces longitudinally thereof; introducing grout under pressure into said longitudinal passages, maintaining said cable under tension and said plank under longitudinal compression while the grout is allowed to set so as to secure said rod members in a substantially untensioned state within their respective lateral passages and effect a stress transmitting bond between said tensioned cable and said plank whereby upon releasing the grip on said leading and trailing ends of the cable and removing said cable looping means from operative association with said cable, the grouted tensioned cable will maintain said plank under compression.

21. The method defined in claim 20 wherein prior to the step of allowing the grout to set, the central portion of the plank formed by the row of blocks is deflected vertically relative to the end portions thereof so as to cause the plank to assume a generally arched configuration and thereby disposed the opposed end portions of the tensioned cable within the transversely centered passage adjacent the upper surface of said transversely centered passage and dispose the central intermediate portion of said cable adjacent the lower surface of said transversely centered passage and said cable is main- 16 tained so disposed relative to said transversely centered passage while the grout is allowed to set.

22. The method defined in claim 21 wherein the plank formed by the row of blocks is supported in its arched configuration by a correspondingly arcuate supporting surface while the grout is allowed to set.

23. The method defined in claim 21 wherein the transversely centered cable containing passage is disposed below the vertically centered longitudinal axis of the plank formed by the row of blocks so that the tensioning of the cable eflects the deflection of the central portion of the plank vertically relative to the end portions thereof so as to cause the plank to'assume the generally arched configuration.

24. The method defined in claim 23 wherein the end faces of each of the blocks converge upwardly so as to limit the amount of vertical deflection of the central portion of the plank relative to the end portions thereof.

25. The method defined in claim 20 which includes the step of providing transversely extending reinforcing members adjacent each end of the plank formed by the row of blocks.

26. The method defined in claim 25 wherein the reinforcing members comprise tie plates positioned between each endmost pair of blocks.

27. A method of simultaneously forming a plurality of prestressed planks comprising the steps of: aligning a plurality of preformed blocks in end to end abutting relationship to provide a row thereof, each of said blocks including a longitudinally extending transversely centered aperture and at least one longitudinally extending lateral aperture transversely spaced from said transversely centered aperture, said apertures being adapted to align with' corresponding apertures in adjacent blocks when said blocks are aligned in end to end abutting relationship so as to define therethrough longitudinally extending.

transversely centered and lateral passages, respectively; positioning at least one spacer member between adjacent blocks at a preselected position along the row so as to provide a separation of the row of blocks into segments forming the planks to be formed, said spacer member being provided with apertures adapted to align with the lateral and transversely centered apertures in adjacent blocks so as to permit communication between the longitudinally aligned longitudinally extending passages in said planks; disposing a rod member in a substantially unstressed state in each longitudinally extending lateral passage in each plank by passing said rod member through the aligned lateral apertures in the row of blocks and said spacer member; disposing a cable in the longitudinally extending transversely centered passage in each plank by passing said cable through the aligned tra11sversely centered apertures in the row of blocks and said spacer member; imparting tension to said cable and exerting compression forces longitudinally of each of the planks formed by the segmented row of blocks; introducing grout under pressure into each of said longitudinally extending passages in each of said planks, maintaining said cable under tension and each plank under longitudinal compression while the grout is allowed to set so as to secure each said rod member in a substantially untensioned state within each plank and effect a stress transmitting bond between said tensioned cable and each plank; and severing said cable and each said rod member at the ends of each segment of the row whereby a plurality of planks are formed wherein the. tensioned cable segment within each plank will maintain said plank underlongitudinal compression.

28. The method defined in claim 27 which includes the step of providing transversely extending reinforcing members adjacent each end of each of the planks formed by the segmented row.

29. The method defined in claim 27 wherein two lengths of tensioned cable are disposed within the longitudinally extending transversely centered passage in each plank.

30. A method of forming a prestressed plank compris ing the steps of: aligning a plurality of preformed blocks in end to end abutting relationship to provide a row thereof so as to define a plank, each of said blocks including a longitudinally extending transversely centered aperture and at least one longitudinally extending lateral aperture transversely spaced from said transversely centered aperture, said apertures being adapted to align with corresponding apertures in adjacent blocks when said blocks are aligned in end to end abutting relationship so as to define longitudinal passages therethrough, each of said passages being disposed below the vertically centered longitudinal axis of said plank; disposing a rod means free of external loading in each passage defined by said lateral apertures; passing a cable means through the transversely centered passage defined by said transversely centered apertures so that at least one length of cable means is disposed Within said transversely centered passage; imparting tension to the cable means to thereby exert compression forces longitudinally of the row of blocks and deflect the central portion of the plank upwardly relative to the ends thereof so as to cause the plank to assume a generally arched configuration and thereby dispose the opposed end portions of the tensioned cable within the transversely centered passage adjacent the upper surface of said transversely centered passage and disposed the central intermedate portion of said cable adjacent the lower surface of said transversely centered passage; introducing grout under pressure into the longitudinal passages, and maintaining the cable means under tension and the row of blocks under compression while the grout is allowed to set so as to secure said rod means in a substantially untensioned condition within the row of blocks and effect a stress transmitting bond between said blocks, said rod means, and said tensioned cable means whereby said tensioned cable means will maintain said row of blocks under compression and said rod means will resist further arching of the plank.

31. A pre-stressed concrete plank comprising in combination in the absence of external load: a plurality of concrete blocks aligned in end to end abutting relationship under longitudinal compression having a longitudinally extending transversely centered passage therethrough and at least one longitudinally extending lateral passage therethrough transversely spaced from said transversely centered passage; a tensioned cable means within said transversely centered passage; a substantially untensioned rod means within each said lateral passage; and stress transmitting bonding means within each of said longitudinally extending passages bonding said substantially untensioned rod means and said tensioned cable means to said blocks.

32. The structure defined in claim 31 wherein the plank includes transversely extending reinforcing means adjacent each end thereof.

33. The prestressed plank defined in claim 32 wherein the reinforcing means comprise tie plates positioned between each endmost pair of blocks.

34. The prestressed plank defined in claim 32 wherein the reinforcing means comprise transversely extending rod members grouted to each endmost block of the plank.

35. The prestressed plank defined in claim 31 wherein the tensioned cable means is positioned adjacent the upper surface of the transversely centered passage at each end of the plank and adjacent the lower surface of said trans versely centered passage at the longitudinal central portion of the plank.

36. The prestressed plank defined in claim 31 wherein at least two lengths of tensioned cable means are carried within the transversely centered passage.

37. The structure defined in claim 31 wherein each of the longitudinal passages are disposed below the vertically centered longitudinal axis of the plank.

38. A prestressed concrete plank comprising in combination: a plurality of concrete blocks aligned in end to end abutting relationship under longitudinal compression having longitudinally extending traversely centered passage therethrough and at least one longitudinally extending lateral passage transversely spaced from said transversely centered passage; a tensioned cable carried within said transversely centered passage, the opposed end portions of said cable being disposed adjacent the upper surface of said transversely centered passage and the central intermediate portion of said cable being disposed adjacent the lower surface of said transversely centered passage; a substantially untensioned rod member carried by each lateral passage; and grout within said longitudinally extending passages securing each said rod member in a substantially untensioned state within its respective lateral passage and bonding said tensioned cable to said plank to thereby maintain said plank under longitudinal compression.

39. An arched prestressed concrete plank comprising in combination in the absence of external load: a plurality of concrete blocks aligned in end to end abutting relationship under longitudinal compression having a pair of longitudinally extending transversely spaced lateral passages and a longitudinal extending transversely centered passage intermediate said lateral passages, each of said passages being disposed below the vertically centered longitudinal axis of said plank; a tensioned steel cable means Within transversely centered passage extending longitudinally therethrough; a substantially untensioned steel reinforcing rod means within each said lateral passage and extending longitudinally therethrough; and grout within each of said longitudinally extending passages bonding said substantially untensioned reinforcing rod means and said tensioned cable means to said blocks.

40. The arched prestressed plank defined in claim 39 wherein at least some of the blocks have end faces which converge upwardly so that the amount of arching caused by compression of the blocks by the tensioned cable is limited.

References Cited by the Examiner UNITED STATES PATENTS 707,201 8/1902 Brinkman 52228 2,413,990 1/1947 Muntz 52223 2,455,153 11/1948 Abeles 52223 2,609,586 9/1952 Parry 52224 X 2,645,115 7/ 1953 Abeles 52229 2,696,729 12/ 1954 Heyden 52229 2,751,776 6/1956 Streblow et al 52228 2,826,800 3/ 1958 Van Buren 52224 X 2,897,570 8/1959 Carper 52227 X 2,925,727 2/1960 Harris et a1 52747 2,949,279 8/ 1960 Eitel 254- 2,949,705 8/1960 Car-per 52--747 3,081,976 3/1963 Carlson et al 25429.5 3,086,273 4/1963 Welborn 52223 X 3,112,097 11/1963 Taylor 254-190 3,172,932 3/1965 Heyden 52227 X 3,194,536 7/ 1965 Simms 25429.5

FOREIGN PATENTS 194,887 1/ 1958 Austria.

207,744 2/1960 Austria.

338,864 11/1930 Great Britain.

541,835 12/ 1945 Great Britain.

664,735 1/ 1952 Great Britain.

685,131 12/1952 Great Britain.

746,617 3/ 1956 Great Britain.

FRANK L. ABBOTT, Primary Examiner.

A. C. PERI-1AM, Assistant Examiner. 

31. A PRE-STRESSED CONCRETE PLANK COMPRISING IN COMBINATION IN THE ABSENCE OF EXTERNAL LOAD: A PLURALITY OF CONCRETE BLOCKS ALIGNED IN END TO END ABUTTING RELATIONSHIP UNDER LONGITUDINAL COMPRESSION HAVING A LONGITUDIALLY EXTENDING TRANSVERSELY CENTERED PASSAGE THERETHROUGH AND AT LEAST ONE LONGITUDINALLY EXTENDING LATERAL PASSAGE THERETHROUGH TRANSVERSELY SPACED FROM SAID TRANSVERSELY CENTERED PASSAGE; A TENSIONED CABLE MEANS WITHIN SAID TRANSVERSELY CENTERED PASSAGE; A SUBSTANTIALLY UNTENSIONED ROD MEANS WITHIN EACH SAID LATERAL PASSAGE; AND STRESS TRANSMITTING BONDING MEANS WITHIN EACH OF SAID LONGITUDINALLY EXTENDING PASSAGES BONDING SAID SUBSTANTIALLY UNTENSIONED ROD MEANS AND SAID TENSIONED CABLE MEANS TO SAID BLOCKS. 